1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device and a method of manufacturing the same, particularly, to a gate structure of a semiconductor device having a nonvolatile memory transistor including a floating gate and a control gate, a selective transistor arranged close to said memory transistor, and a peripheral circuit mounted on the same chip.
2. Description of the Related Art
Known is a flash memory having a memory transistor including a floating gate and a control gate, a selective transistor arranged close to the memory transistor, and a peripheral circuit for driving the memory transistor and the selective transistor mounted to the same chip. A typical flash memory is called a NAND type flash memory. The NAND type flash memory includes a plurality of memory transistors connected in series, a selective transistor arranged close to the both edge portions of the memory transistor, and a peripheral circuit transistor for driving the memory transistor and the selective transistor. The region in which are arranged the memory transistors is called a memory cell array region, the region in which is arranged the selective transistor is called a selective gate region, and the region in which is arranged the peripheral circuit transistor is called a peripheral circuit region.
The method of preparing the flash memory of this type includes, for example, the steps of forming a gate insulating film on a semiconductor layer, depositing a polycrystalline silicon (polysilicon) film providing the floating gate of the memory transistor on the gate insulating film, and forming an element isolating region. In this case, a gate electrode of a two layer structure consisting of a floating gate and a control gate is present in at least a portion of the selective gate region and the peripheral circuit region as in the memory cell array region. It should be noted that it is necessary for the transistor in the selective gate region and the peripheral circuit region to be electrically connected to an upper wiring by withdrawing the floating gate. A conventional semiconductor device of this type will now be described.
FIG. 46A is a plan view showing the memory cell array region and the selective gate region of the semiconductor device according to the first prior art. FIG. 46B is a plan view showing the peripheral circuit region of the semiconductor device according to the first prior art. FIG. 47A is cross sectional view of the semiconductor device along the line XXXXVIIA-XXXXVIIA shown in FIGS. 46A and 46B. FIG. 47B is cross sectional view of the semiconductor device along the line XXXXVIIB-XXXXVIIB shown in FIG. 46A. The first a prior art is disclosed in Japanese Patent Disclosure (Kokai) No. 11-163304.
As shown in FIGS. 46A, 46B, 47A and 47B, a first insulating film 12 is formed on a semiconductor layer 11, and a first floating gate electrode layer 13a is formed on the first insulating film 12. Then, an element isolating groove is formed and the element isolating groove thus formed is filled with an insulating film. The insulating film if planarized until the first floating gate electrode layer 13a is exposed to the outside so as to form an element isolating region 15. Then, a second floating gate layer 13b consisting of polysilicon is formed on the first floating gate layer 13a and the element isolating region 15, followed by patterning the second floating gate electrode layer 13b by a lithography and an etching. Consequently, an open potion 50 is formed on the element isolating region 15 of the memory sell array region and the open potion 50 isolated the second floating gate electrode layer 13b. Further, a second insulating film 16 is formed on the second floating gate electrode layer 13b and the element isolating region 15, followed by forming a control gate electrode layer 18 on the second insulating film 16. After the control gate electrode layer 18, the second insulating film 16 and the first and second floating gate electrode layers 13a and 13b are patterned, a third insulating film 19 is formed on the entire surface of the semiconductor layer 11. Further, after a contact hole is formed in the third insulating film 19, a wiring 21 connected to the contact hole 20 is formed. As a result, the wiring 21 is connected to the control gate electrode layer 18 via the contact hole 20 in the memory cell array region, and the wiring 21 is connected to the first and second floating gate electrode layers 13a, 13b via the contact hole 20 in the selective gate region and the peripheral circuit region.
The semiconductor device according to the first prior art described above comprises a floating gate of the double layer structure consisting of the first and second floating gate electrode layers 13a and 13b. In the floating gate of the particular construction, the first floating gate electrode layer 13a is self-aligned with the element isolating region 15, and the second floating gate electrode layer 13b is pulled up onto the element isolating region 15. However, the first prior art described above gives rise to problems.
First of all, in the memory cell array region, it was necessary to set the width P of the open portion 50 such that the open portion 50 is not buried with the second insulating film 16. It was also necessary to ensure an aligning allowance Q between the open portion 50 and the element region 10 in the lithography. However, it was difficult to finely adjust the open portion 50 because of the limit in the resolution of the photoresist in patterning the open portion 50. As a result, it was difficult to achieve the fineness beyond a certain level, with the result that it was difficult to make the memory cell finer.
On the other hand, in the peripheral circuit region, the contact hole 20 is formed on the element isolating region 15, making it possible to avoid the damage done to the element region. However, the element region is formed a long distance away from the connecting portion 25 between the second floating gate electrode layer 13b and the contact hole 20. Therefore, since the second floating gate electrode layer 13b is formed of in general an electrode material having a high resistivity such as polysilicon, the delay caused by the resistance is increased so as to lower the performance of the element. It should also be noted that, if the second floating gate electrode layer 13b is allowed to extend over the element isolating region 15, a capacitance coupling is formed between the semiconductor layer 11 and the floating gate with the insulating film of the element isolating region 15 interposed therebetween, leading to an increased RC delay.
Particularly, when it comes to the selective transistor of the NAND type flash memory, the increase in the RC delay described above is a serious problem to be solved. The contact to the second floating gate electrode layer 13b is formed as required for several cells within the memory cell array. The contact portion requires an area so as to increase the area of the memory cell array. Also, since the contact hole 20 can be formed in only a part of the memory cell array, the contact hole 20 is connected to the transistor via the second floating gate electrode layer 13b formed of polysilicon having a high resistivity. It follows that the problem of the RC delay time to the transistor positioned remote from the contact hole 20 is rendered serious. It should be noted that the increase in the delay time of the selective transistor adversely affects the reading speed of the memory cell.
FIG. 48A is a plan view showing the memory cell array region and the selective gate region of the semiconductor device according to the second prior art. FIG. 48B is a plan view showing the peripheral circuit region of the semiconductor device according to the second prior art. FIG. 49A is cross sectional view of the semiconductor device along the line XXXXIXA-XXXXIXA shown in FIGS. 48A and 48B. FIG. 49B is cross sectional view of the semiconductor device along the line XXXXIXB-XXXXIXB shown in FIG. 48A. The second prior art is intended to avoid the problem inherent in the first prior art that it is difficult to make the memory cell portion finer.
As shown in the drawing, a first insulating film 12 is formed on a semiconductor layer 11, and a floating gate electrode layer 13 is formed on the first insulating film 12. Then, an element isolating groove is formed, followed by filling the element isolating groove with an insulating film. An element isolating region 15 is formed by planarizing the insulating film until the surface of the floating gate electrode layer 13 is exposed to the outside. Then, an upper portion of the element isolating region 15 in the memory cell array region and the selective gate region is removed so as to allow the upper surface of the element isolating region 15 in the memory cell array region and the selective gate region to be positioned lower than the upper surface of the floating gate electrode 13. Further, a second insulating film 16 is formed on the floating gate electrode layer 13 and the element isolating region 15, followed by removing the second insulating film 16 in the peripheral circuit region and the selective gate region. In the nest step, a control gate electrode layer 18 is formed on the second insulating film 16, the floating gate electrode layer 13 and the element isolating region 15, followed by patterning the control gate electrode layer 18, the second insulating film 16 and the floating gate electrode layer 13. After the patterning step, a third insulating film 19 is formed on the entire surface of the semiconductor layer 11, followed by forming a contact hold 20 in the third insulating film 19. In the nest step, a wiring 21 connected to the contact hole 20 is formed.
In the semiconductor device according to the second prior art described above, it is unnecessary to ensure an aligning allowance Q in the lithography, which is required in the first prior art, so as to make it possible to miniaturize the memory cell. Also, since the control gate electrode layer 18 is deposited after removal of the selective gate region and the second insulating film 16 of the peripheral circuit region, the limitation in the position of the contact hole 20 can be eliminated even if the circuit is separated such that the floating gate is left unremoved in only the element region 10. However, the second prior art gives rise to the problem as described below.
First, the second insulating film 16 is interposed between the floating gate electrode 13 and the control gate electrode layer 18 in the gate in the memory cell array region. However, the second insulating film 16 is not interposed between the floating gate electrode 13 and the control gate electrode layer 18 in the gate in the peripheral circuit region and the selective gate region. In other words, the memory cell array region, the peripheral circuit region and the selective gate region differ from each other in the laminate structure of the gate. As a result, in forming the gate, it is necessary for the memory cell array region, the peripheral circuit region and the selective gate region to be different from each other in the etching conditions, giving rise to the problem that it is impossible to form simultaneously the gates in the memory cell array region, peripheral circuit region and the selective gate region.
It should also be noted that, if it is impossible to form simultaneously the gates in the memory cell array region, the peripheral circuit region and the selective gate region, the electrode layer is left unremoved in the boundary portion between the memory cell array region, the peripheral circuit region and the selective gate region. Also, it is necessary to ensure a sufficient allowance region in order to prevent the semiconductor layer from being dug by the etching treatment performed twice. In order to process accurately both the memory cell array region, the peripheral circuit region and the selective gate region differing from each other in the laminate structure, it is necessary to ensure various allowances in the boundary portion, leading to an increase in the chip area. Particularly, in the construction of the NAND type flash memory, it is necessary to diminish the distance D between the memory cell and the selective transistor as much as possible in order to increase the degree of integration of the memory cell array, as shown in FIG. 48A. What should be noted is that, if an allowance is provided in the boundary portion, the degree of integration is markedly lowered.
As described above it was very difficult to avoid the resistance delay in the peripheral circuit region and the selective gate region while miniaturizing the memory cell array region and to form simultaneously the gates in the memory cell array region, the peripheral circuit region and the selective gate region in the semiconductor device according to each of the first and second prior arts.